Intensive thinning needed to restore some western forests

Intensive thinning needed to restore some western forests

20 August 2011

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USA — New research by the U.S. Forest Service appears to confirm that massive thinning efforts are needed to restore some measure of fire resistance to western ponderosa pine and Douglas fir forests where decades of fire suppression upset the ecological balance.

The study of fuel treatment effectiveness concluded that intense thinning treatments leaving between 50 and 100 trees per acre are the most effective in reducing the probability of crown fires in the dry forests of the western United States, including areas in Arizona and New Mexico that experienced their largest fires in modern history.

In Colorado, the results of the study are valid for some Front Range forests, but no so much for the higher-elevation lodgepole pine and spruce and fir forests which have evolved with a different fire regime.

The results of the study were published in a recent issue of the Canadian Journal of Forest Research. Forest Service Chief Tom Tidwell said it provides a scientific basis for establishing quantitative guidelines for reducing stand densities and surface fuels. The total number of optimal trees per acre on any given forest will depend on species, terrain and other factors.

“This study proves once again that an ounce of prevention equals a pound of cure,” said U.S. Forest Service Chief Tom Tidwell. “Thinning dense forests reduces the impacts of the catastrophic wildfires we’ve already seen this year and expect to see more and more of in the future. This work helps protect communities, provides jobs and promotes overall better forest health.”

Excluding fires from forests that depended on low intensity ground fires to clear out brush and ladder fuels has resulted in densely packed stands and a buildup of forest-floor fuels that can lead to large, continuous crown fires. Crown fires are of particular concern to managers because they are challenging to suppress and are capable of causing widespread mortality in stands.

The importance of thinning was illustrated by the recent Wallow fire in Arizona, which burned more than 538,000 acres. Although 38 structures burned, a system of fuel treatments developed cooperatively by federal, state and local governments, as well as private citizens, successfully reduced fire behavior and allowed firefighters to protect thousands of structures and, in many places, halt the spread of the fire.

“Most forest managers understand that dry Western landscapes need to be heavily thinned to significantly reduce the threat of crown fires, and our findings now give a sense of just how much thinning is required,” said Morris Johnson, the study’s lead and a research fire ecologist based at the station’s Pacific Wildland Fire Sciences Laboratory. “We found that thinning at this level reduced tree density, raised the canopy base height, and reduced canopy density.”

To test how effective fuel treatments in the Western states are in reducing the probability and severity of crown fires, Johnson, along with University of Washington researcher Maureen Kennedy and station research biologist David L. Peterson simulated the effects of four types of thinning and surface fuel treatments in dry forest types in 11 Western states.

The models included weather and fuel conditions data to simulate fire conditions in 45,162 forest stands depicting the crown fire hazard and potential fire behavior based on thinning densities leaving 300, 200, 100, and 50 trees per acre.

“This kind of simulation modeling allows us to evaluate multiple treatment types across a large number of forest stands and conditions in diverse geographic areas,” Johnson said. “It would be almost impossible to conduct a study like this on the ground.”

Their simulations suggested that the effectiveness of fuel treatments in the West depends on thinning intensity, with the most intense treatments they studied, which leave 50 to 100 trees per acre, being more effective in reducing the threat of crown fires than less-intense treatments. Thinning to this level, along with the removal of post-treatment debris known as “slash,” made conditions unfavorable for crown fire initiation and reduced the probability of active crown fire.

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